1. Field of the Invention
The present invention relates to fabrication of items with small features, and, more paticularly, to lithographic methods of fabrication such as are used in semiconductor integrated circuits.
2. Description of the Related Art
Semiconductor-based electronic integrated circuits have progressively shrunk in feature size and increased in complexity since their invention in 1958, and currently mass-produced integrated circuits such as computer memory chips have feature sizes on the order of 1 .mu.m. Such integrated circuits are typically fabricated using photolithography which patterns a layer of photoresist (a radiation sensitive material) on the in-process integrated circuit by exposure of the photoresist to masked radiation such as visible light or ultraviolet light. The radiation causes a chemical change in the photoresist, and the exposed areas of the photoresist may then be selectively removed (positive photoresist) or retained (negative photoresist) by contact with a solvent. The patterned photoresist is then used as a mask in a step (such as etching, deposition, ion implantation, etc.) of the fabrication process for the integrated circuit.
FIGS. 1a-c illustrate in cross sectional elevation views the formation and use of photoresist as an implantation mask. As shown in FIG. 1a photoresist (positive) 102 is applied to substrate 104, photoresist 102 is typically 1 .mu.m thick. Next, ultraviolet radiation 106 is through pattern mask 108 and exposes photoresist 102 in the pattern of mask 108; see FIG. 1b. The exposed portion of photoresist 102 is then removed (photoresist 102 is developed) by dissolution in a solvent, and the developed photoresist 102 may then be used as a mask for ion implantation of dopants 110 to form doped regions 112 in substrate 104; see FIG. 1c.
The feature size obtainable using photoresist lithography is limited by diffraction effects, with resulting dimensions having a lower limit of about 0.1 .mu.m even if ultraviolet light is used for the photoresist exposure. That is, in the exposure step illustrated in FIG. 1b, the wavelength of light 106 is comparable to the size of the openings in mask 108 and the light passing through the openings is severely diffracted. In contrast, the use of resists sensitive to short wavelength entities such as electrons, ions, or x-rays will eliminate this diffraction limitation, but then the limitations of the resist material itself become important. For example, electron beams can be focussed down to a spot size of the order of 10 .ANG., and can be used to directly write on (expose) a resist layer without the use of a pattern mask. The typical electron beam resist, polymethylmethacrylate (PMMA), is exposed by the incident electrons breaking bonds (e.g., carbon-carbon bonds) to make the PMMA more soluble in a developer such as methylisobutylketone (MIBK). A spot size of less than 10 .ANG. for the incident electron beam can be formed and accurately controlled with digital and analog techniques; however, the incident electrons typically have energies on the order of 20 KeV and create secondary electrons as they inelastically scatter in the PMMA. These secondary electrons have energies up to about 100 eV and can also break carbon-carbon bonds. The secondary electrons have a range of up to about 100 .ANG. from the incident flux, and these account for the feature size limitation in PMMA of about 125 .ANG.. Molecular size and statistical effects may also limit feature size. See R. Howard et al, Nanometer-Scale Fabrication Techniques in 5 VLSI Electronics: Microstructure Science, pp. 150-153 (Academic Press 1982). FIGS. 2a-b are cross sectional elevation views of Monte Carlo simulations that illustrate the spreading of the incident electrons and the secondary electrons in a layer of PMMA.
Statistical effects due to fluctuations in the flux of incident electrons and fluctuations in the number of secondary electrons also affect the feature size limitation for electron beam direct write exposure of resist. Further, the incident electrons may also scatter off of the substrate back into the resist and further expose the resist. The overall result is a difficulty in achieving feature sizes approaching 100 .ANG..
Attempts to overcome the feature size limitations of standard lithography include use of thin resist layers (to avoid secondary electron spread as in FIG. 2b), thin substrates (to avoid backscattering of incident electrons), multilayer resists (effectively thin resist plus lower layers absorb backscatter). However, these approaches are not yet useful in high volume manufacturing.
J. Cleaver et al, A Combined Electron and Ion Beam Lithography System, 3 J. Vac. Sci. Tech. B 144 (1985) theoretically analyze a combination of ion beams and electron beams in a single machine for microlithography.